These studies were undertaken to examine the relationship between tamoxifen and sex steroid hormones in rat uterine morphology and the effect of tamoxifen on sex steroid hormone levels, implantation and myometrial contraction. The results obtained were as follows : 1) The increase in height of the luminal epithelium caused by tamoxifen treatment was blocked by progesterone. The increase in height of luminal epithelium caused by $estradiol-17{eta}$ treatment was blocked by tamoxifen. 2) When a single dose of tamoxifen(10, 20, $40{mu}g$) was given on Day 2 of pregnancy, implantation was prevented. Plasma $estradiol-17{eta}$ level fell in a dose-dependent manner but plasma progesterone level was constant. 3) In vitro, tamoxifen decreased rat uterine contractility in a dose-dependent manner.